1. Field of the Invention
The present invention relates to a multi-tank dishwasher with at least one washing zone for acting upon a wash batch with washing water in a washing chamber and comprising at least one washing water storage tank, at least one filter housing and a backwash device. The invention relates, furthermore, to a method for operating a multi-tank dishwasher, the method having normal operation and backwash operation.
2. Description of the Background Art
To clean wash batches, nowadays the commercial sector not only employs single-chamber automatic dishwashers, but also multi-tank dishwashers, in which the wash batch to be cleaned is transported by means of a conveying device through the various zones of the dishwasher. Multi-tank dishwashers of this type comprise, as a rule, at least one washing zone, at least one rinsing-clear zone and, optionally, a drying zone. In this case, the wash batch to be cleaned runs in succession through said treatment zones. For this run, multi-tank dishwashers may be designed, for example, as belt transport or basket transport machines. Both versions have in common the fact that the wash batch is transported, for example continuously, through the individual treatment zones by the transport means.
At the start of operation, in the multi-tank dishwasher, the washing water tanks of the washing zone are filled with fresh water and heated. Furthermore, cleaning agent is added to the washing water. In this case, a plurality of washing zones may be arranged one behind the other, the washing waters of which have, for example, different temperatures with different cleaning agents and/or with different cleaning agent concentrations.
A washing zone is normally designed as a chamber and typically has an inlet orifice and an outlet orifice. The wash batch is transported through these orifices and through the respective washing zone by means of the conveying device. Conventionally, a washing zone has a washing water tank and a circulating pump which sucks in washing water from the washing water tank and sprays it onto the wash batch via a spray system assigned to the washing zone. In this case, the dirt adhering to the wash batch is removed.
The washing water subsequently flows, together with the washed-off dirt, back into the washing water tank again. In this case, the washed-off dirt is filtered out of the washing water by means of a sieve system. Conventionally, sieve plates with hole diameters of 2 mm to 4 mm are used here, which cover the entire washing water tank. Dirt fractions which are smaller than the hole diameter are circulated together with the washing water.
Subsequently, in the multi-tank dishwasher, cleaning agent and dirt residues lying loosely on the wash batch are washed off by means of hot fresh water in one or more rinsing-clear zones. A rinsing-clear agent may in this case be added additionally to the hot fresh water. The fresh water or rinsing-clear water is then supplied usually completely or partially to a washing water tank of a washing zone, in order to dilute the dirt fractions located there in the washing water tank. After running through one or more rinsing-clear zones, the wash batch optionally runs through one or more drying zones in which the wash batch is dried.
The abovementioned sieve plates in the at least one washing zone often have the disadvantage that the dirt filtered out from the washing water remains lying on these and is not actively removed from the washing zone. By the washing water falling down, this dirt may be further comminuted and then likewise passes, with a time delay, into the washing water of the washing zone and increases the dirt content of the washing water there. This is a disadvantage particularly since the respective cleaning capacity of a washing zone typically decreases with the quantity of dirt in the washing water.
The dirt content of the washing water of the washing zone may be counteracted by increasing the fraction of rinsing-clear water supplied to the washing zone from the rinsing-clear zone. The disadvantage of this, however, is that, in this method, the consumption of cleaning agent which has to be added to the washing water rises in the same ratio. Operating costs and environmental pollution increase as a result.
A further possibility of reducing the dirt fraction in the washing water of the washing zone is to use sieve plates of smaller hole diameter in the washing zone. A smaller diameter of the sieve holes increases the risk, however, that the sieve plate becomes blocked. A partial or complete blockage of the sieve plate means, however, that sufficient washing water no longer flows back into the washing water storage tank, with the result that the pump pressure with which the wash batch is acted upon with washing water decreases and therefore the washing action is adversely influenced.
If a certain dirt content in the washing water is overshot in the washing zone in spite of the normally employed sieve plates and the supply of rinsing-clear water from the rinsing-clear zone, then, as a rule, the entire washing water tank contents have to be changed. This entails costs in terms of water and sewage. Furthermore, this means, as a rule, a stoppage time for the dishwasher, and also increased personnel costs for cleaning the respective washing zone and costs for heating energy to heat up the washing water to the preset temperature of usually 60° C., as well as costs for the new cleaning agent which has to be added to the washing water again.
From the sector of single-chamber dishwashers, filter systems are known in which the washing water storage tank is likewise covered by a sieve plate. In one region of the sieve plate, however, a coarse sieve is used, which is followed by a fine sieve. The washing water is likewise circulated within the “washing” program steps. Part of the washing water, after the latter has run through the coarse sieve, flows through the fine sieve. The fine dirt fractions of the washing water are also retained in the latter. When the washing water is changed, a sewage pump conveys the washing water, together with the fine and coarse dirt, out of the fine sieve into the sewage.
A device of this type is described in DE 24 51 822 C2. The device described in this publication discloses a collecting bowl for a dishwasher, with a first suction-intake space connected to a lye pump and with a second suction-intake space connected to a circulating pump. The first and the second suction-intake space are in this case connected to one another by means of a fine filter sieve. In a suction-extraction step, washing liquid is suction-extracted from the dishwasher, washing liquid being drawn off from both suction-intake spaces by means of the lye pump.
DE 14 28 358 illustrates a further design of a single-chamber dishwasher. This known embodiment additionally has spray nozzles which spray onto a fine filter from outside, with the result that the fine filter is cleaned and dirt residues can be removed by means of a sewage pump. A similar self-cleaning principle, in which a filter element is cleaned by means of a spray nozzle, is also disclosed in EP 0 976 359 A1 and DE 69 820 625 T2. EP 1 256 308 A2 discloses a device which, in addition to a spray nozzle, also additionally has a dirt comminutor.
The devices known from the prior art typically function to the effect that the washing operation is interrupted for cleaning the fine sieve. In this case, a separate program step of fine sieve cleaning is carried out, with the washing water pump switched off. This is due, in particular, to the fact that in the devices described, in washing operation a washing liquid constantly flows through the fine sieve in a filter direction, so that the fine sieve is not directly accessible for cleaning (for example spraying from outside). This interruption in the washing operation for cleaning the sieves used thus entails an additional amount of time, during which the respective washing chamber cannot be employed.
The described embodiments known from the prior art therefore have in common the fact that they are designed for use in single-tank water-changing machines and cannot readily be transferred to a continuously operating washing zone of a multi-tank dishwasher. In a multi-tank dishwasher, a washing water circulating pump operates permanently, relatively large washing water quantities per minute being circulated, and therefore relatively large quantities of washing water flowing constantly into the fine filter. In operation of this type, a cleaning of the sieves used, for example by spraying, particularly in a separate cleaning step, can be implemented only with difficulty in technical terms and, particularly in the case of industrial applications, would cause a troublesome and costly interruption in washing operation.